Wideband tapered slot and horn antennas—commonly known as “Vivaldi slot” or “Vivaldi horn antennas”—are known as having an advantage of wideband bandwidth, often 10:1 or more bandwidth, with the ability to superpose a second Vivaldi antenna at a right angle, thereby capturing two orthogonal polarizations of electromagnetic waves. Examples of prior Vivaldi antennas are shown and described in U.S. Pat. Nos. 6,043,785, 5,519,408, 5,036,335, and 4,855,749, among others.
FIG. 1, derived from U.S. Pat. No. 5,036,335, shows an exponentially tapered slot Vivaldi antenna 102 defined by a metallized layer 105 on one main face of a substrate 104. The antenna 102 has a conventional feed arrangement comprising a stripline defined by a narrow conductor 101 (dotted) on one main face of the substrate 104 and a slot line 103 extending from the narrower end of the slot antenna 102 to form a balun by crossing over one another at right angles at a point D. The strip line 101 terminates in an open-circuit and extends beyond the slot line 103 by a distance λm/4. The slot line 103 terminates in a short-circuit and extends beyond the stripline 101 by a distance λs/4. The wavelengths λm and λs are respectively the guide wavelength in the stripline 101 and the slot line 103 at the operating frequency of the antenna. Thus, at the cross over point D the stripline 101 is effectively short-circuit and the slot line 103 is effectively open-circuit. This form of balun has been observed to have an inherent narrow bandwidth characteristic.
FIG. 2 shows a photograph or photo-based drawing of a prior-art Vivaldi slot antenna 200 similar to that depicted in FIG. 1. As shown, the Vivaldi slot antenna 200 includes a conductive surface 202 defining a tapered (exponentially) slot 204. Feed line 206 is shown along with stub termination 208, which can be used for impedance matching.
A significant disadvantage of Vivaldi antennas is that they have a large size which often makes them unwieldy, impractical, or unusable for many applications, particularly those where size or form factor is a primary consideration or design constraint. At lower frequencies of operation, with commensurate longer wavelengths, the requisite size of a typical Vivaldi antenna is driven upwards. Such increases in size may be deleterious or impossible to accommodate for some antenna applications. Prior art Vivaldi antennae have also been observed to suffer from degraded gain performance at the low end of their operational passbands.